1. Field of the Invention
The present invention relates to a method and apparatus for plasma processing a substrate, and more particularly to a method and system for monitoring a plasma process in order to determine an endpoint to the plasma process.
2. Description of Related Art
During semiconductor processing, a (dry) plasma etch process can be utilized to remove or etch material along fine lines or within vias or contacts patterned on a substrate. The plasma etch process generally involves positioning a semiconductor substrate with an overlying patterned, protective layer, for example a photoresist layer, in a processing chamber. Once the substrate is positioned within the chamber, an ionizable, dissociative gas mixture is introduced within the chamber at a pre-specified flow rate, while a vacuum pump is throttled to achieve an ambient process pressure.
Thereafter, plasma is formed when a fraction of the gas species present are ionized by electrons heated via the transfer of radio frequency (RF) power either inductively or capacitively, or microwave power using, for example, electron cyclotron resonance (ECR). Moreover, the heated electrons serve to dissociate some species of the ambient gas species and create reactant specie(s) suitable for the exposed surface etch chemistry. Once the plasma is formed, selected surfaces of the substrate are etched by the plasma. The process is adjusted to achieve appropriate conditions, including an appropriate concentration of desirable reactant and ion populations to etch various features (e.g., trenches, vias, contacts, etc.) in the selected regions of the substrate. Such substrate materials where etching is required include silicon dioxide (SiO2), low-k dielectric materials, poly-silicon, silicon, and silicon nitride.
As the circuit ground-rules continue to shrink, the sensitivity of materials to the plasma etch process increases and, hence, the effective determination of the endpoint for the etch process becomes evermore critical. The endpoint techniques utilized can be broadly classified into two categories, namely: (i) optical and (ii) electrical-based techniques.
Optical emission spectrometry (OES) has been commonly used to determine the endpoint of an etch process. This approach relies on the monitoring of the emission of an excited chemical species present in the etch process. However, there are limitations to this technique, as not all excited chemical species are emissive or strong emitters. Therefore, the application of OES to etch processes is limited only to certain etch chemistries. Furthermore, the ability to decipher the endpoint is complicated by the composition of the etch stop layer, since the etch stop layer can introduce chemical species that emit in the spectral region of interest. Since in these instances, the endpoint can not be accurately determined, a finite loss of the etch stop layer is inevitable, which may, in turn, damage the underlying device. Furthermore, this technique requires a high signal-to-noise ratio. For example, in processes having low pressure or low pattern factors, even when emitting chemical species exist, the reliable collection of the emission signal becomes an issue rendering the technique ineffective.
Interferometric methods have also been used to monitor endpoint. With this technique, a monochromatic beam (e.g., from a He—Ne laser source at 632.8 nm) is incident on the substrate being etched. The interference patterns, which evolve from the dynamically etched film and the stack underneath, are used to calibrate endpoint. Variations in the topography and film thicknesses, which are inherent to the process flow, modify the interference patterns and thus affect the reliability of this method. Moreover, the sampling of data on the substrate is limited to the area exposed to the incident beam. As with OES, an optical window, through which optical signals are passed, is required to preserve vacuum conditions. As an etch process or processes proceed, the transmission properties for the optical window degrade due to the accumulation of process residue which leads to a loss of signal.
Another technique for determining the endpoint of an etch process includes monitoring the self-bias potential (e.g., see U.S. Pat. No. 6,517,670, U.S. Pat. No. 6,297,165, U.S. Pat. No. 5,198,072, JP01151234A, U.S. Pat. No. 6,562,187, U.S. Pat. No. 6,811,362). The substrate in contact with the plasma develops a negative direct current (DC) potential due to a difference in the flux of electrons and ions arising from the differences in mobility. The material of interest being etching on the substrate contributes to the plasma impedance of the circuit, and thus to the bias (DC) potential. Upon completion of the etch process, the circuit impedance changes resulting in a shift in the DC potential which can be utilized to identify endpoint for the process. In yet another example, Japanese patent JP59043881 inspects the DC bias through the substrate. However, this approach causes a dilution of the endpoint signal because only current propagating through the substrate is sampled.